Photonic crystals may respond to incident light in a unique manner, exhibiting the presence of a “photonic bandgap” (PBG) in addition to tunable transmittance or reflectance. While light waves can propagate through a photonic crystal with a periodic dielectric structure, a periodic dielectric function (or index of refraction), called the photonic bandgap (PBG), may forbid propagation of light in certain directions and for certain ranges of wavelengths. Photonic crystals may be suitable for use in optical devices such as optical limiters, switches, diodes, and transistors.
PBG structures may serve as an alternative to conventional transparent materials. One-dimensional PBG materials are layered, periodic structures that include materials with different dielectric constants, and hence optical constants. The periodicity in the dielectric contrast for such structures gives rise to a PBG, the size of which depends on the dielectric contrast and the number and dimension of periods within the structure. A set of degenerate standing wave solutions to Maxwell's equations is obtained in the band edge regions such that, in the lower band edge, localization of the electric field intensity (of the incident electromagnetic wave) is maximized in the layer with higher dielectric constant (i.e., a higher refractive index), and vice versa for the higher band edge.
Metals, being conductive materials, with their free carrier plasma resonance or absorption frequencies usually inside the visible spectrum, are typically highly reflective and absorptive in all wavelength ranges of interest. Due to these characteristics, metals may be suitable for electromagnetic shielding applications, but less suitable for transparent conducting applications. However, when metal films are combined with dielectric materials that are transparent in a layered composite structure, the transparency or transmittance of the metal films can be enhanced by using a mechanism that has become known as optical resonant tunneling. In particular, if the thickness of the layers of the metal films and the dielectric materials is chosen such that, at the metal absorption frequency, the light and electric field is concentrated in the non-absorbing dielectric layers (e.g., when the metal absorption frequency is equal to the lower band edge frequency of the layered structure), then absorption by the metal films can effectively be suppressed. While these structures may conduct along the plane of the metal layers, out-of-plane or transverse conductance may be limited because of the presence of the insulating dielectric layers between the metal layers.